Garden Irrigation Calculator

Direct answer

Multiply the irrigated area by the water depth and 0.6233 gallons per square foot per inch, then divide by the delivery efficiency you choose. EPA WaterSense documentation uses the same 0.6233 conversion factor within a broader landscape-water-budget method; this calculator is a transparent volume scenario, not a plant-watering prescription.

Variables and formula

• Garden area: the measured irrigated area in square feet.
• Water depth: inches selected for one irrigation event.
• System efficiency: the share of applied water expected to reach the planned area.
• Events per month: the planned event count for the monthly total.

Gallons to apply = Garden area * Water depth * 0.6233 / System efficiency

Reproducible example

For 500 sq ft at 0.75 in per event and 70% efficiency, 500 * 0.75 * 0.6233 / 0.70 = 333.91 gallons per event. Four events produce 1,335.64 gallons for the month.

Interpretation

Use the number to plan or compare a stated scenario. Local rainfall, soil, plant type, slope, restrictions and system condition should determine whether and when irrigation is appropriate.

Assumptions and limitations

• The area is a reasonably uniform irrigation zone.
• The selected efficiency is an assumption rather than a measured audit.
• This calculator does not diagnose plant health or prescribe a watering schedule.

Common mistakes

• Applying one depth or frequency to every plant and soil type.
• Forgetting to convert efficiency from a percentage to a decimal rate.
• Treating estimated applied gallons as proof of water reaching roots.

Assumptions

• The garden area and chosen depth describe a single reasonably uniform irrigation zone.
• The entered efficiency is a planning assumption, not a measured system audit.
• Plant needs, rainfall, soil, slope, weather, local restrictions and runoff are not predicted.

Sources and methodology

CalcMotive publishes the formula and assumptions so you can decide whether the estimate fits your use case. See our methodology standards.

• Revised WaterSense Water Budget Approach; U.S. Environmental Protection Agency; accessed Aug 3, 2026